Deuteronomy 21:1

IF one be found slain in the land which the Lord thy God giveth thee to possess—lying in the open field; and it is not known who smote him; thine elders, and thy judges, shall come and measure the distance to the cities around the slain;

And if one be found slain with the sword in the land, which the Lord thy God gives thee to inherit, having fallen in the field, and they do not know who has smitten him;